The invention relates to a method for centering and clamping a workpiece, in particular an articulated shaft, in a balancing machine and a chuck for carrying out the method.
For balancing, workpieces must be rotatably mounted in a balancing machine in such a manner that the rotation axis which the workpieces each assume in their subsequent operating position corresponds as precisely as possible to the rotation axis of the balancing machine. When the workpieces are received in a chuck, in order to determine the rotation axis, the workpieces are generally provided with a mostly cylindrical centering surface and with a rotationally symmetrical mostly planar contact surface which extends radially and by means of which the rotation axis of the workpiece is determined. In the chuck there are provided receiving members which co-operate with these surfaces, for example, a radially supporting centering clamping member, axially supporting clamping surfaces or the like, to which the workpiece is securely clamped by means of clamping elements. When securely clamping, it may be necessary for the clamping at the centering surface of the workpiece and the clamping at the radial contact surface not to be able to take place at the same time, so that particular attention must be paid in order to prevent the workpiece from being clamped in an inclined manner.
For balancing, articulated shafts are generally secured in a horizontal position, by the ends thereof, to two spindles of a balancing machine, each spindle having a chuck in which the flange which is arranged at the end of the articulated shaft is centered and clamped. In this instance, chucks are used which are configured in such a manner that the flange of the articulated shafts becomes centered in the conical clamping device thereof, which engages in a hole of the flange, and is radially clamped with significant actuating force. Subsequently, the flange is pressed with a radial contact surface, by means of connecting rods and clamping elements which surround the flange from the outer side, against a clamping surface of the chuck. In this instance, it may be the case that, owing to the radial clamping with significant actuating force which has been previously carried out, the axial pressing force of the clamping elements is no longer sufficient to press the contact surface of the flange in a uniform manner against the clamping surface of the chuck. The flange may therefore be in an inclined position which results in an inadmissibly significant and irreproducible error during the balancing operation.
An object of the invention is to provide a method of the type mentioned in the introduction which prevents clamping errors and which ensures very precise clamping of the workpieces. The method is further intended to be able to be carried out automatically. Furthermore, an object of the invention is to provide a chuck for a balancing machine which ensures automatic centering and clamping of workpieces, in particular articulated shafts, with a high level of precision.
According to the invention, the method for achieving the object set out involves the steps of positioning a workpiece with a centering surface on a centering clamping member of a chuck, pressing a radial contact surface of the workpiece against a clamping surface of the chuck with a first force which still allows radial movement of the workpiece, centering the workpiece by clamping the centering clamping member against the centering surface of the workpiece, and pressing the radial contact surface of the workpiece against the clamping surface of the chuck with a greater second force which securely clamps the workpiece. Preferably, the second force is at least three times greater than the first force.
In accordance with the method according to the invention, the planar radial contact surface of the workpiece is first pressed with a relatively small first force against the clamping surface of the chuck. A parallel orientation of the rotation axes of the chuck and workpiece is thereby achieved, but with the workpiece still being able to be radially displaced for the subsequent centering operation. Owing to the subsequent clamping of the centering clamping member, the workpiece can therefore be precisely centered, the contact between the planar contact surface and the clamping surface and consequently the correct orientation of the rotation axis of the workpiece being maintained owing to the axial effect of the first force. After the radial clamping of the centering clamping member, which can also be carried out with a high degree of force, the rotation axis of the workpiece is orientated in an optimal manner relative to the rotation axis of the chuck so that owing to the subsequent axial clamping of the workpiece with the significantly greater second force, the workpiece can be definitively fixed in the chuck. Inclined clamping of the workpiece is prevented in this instance.
In accordance with the invention, one advantageous device for carrying out the method according to the invention comprises a centering clamping member with a centering portion having a variable diameter, a radially extending clamping surface, which is adjacent to the centering clamping member, a movable clamping element for axially pressing the workpiece against the clamping surface, a first spring element via which the centering clamping member can be moved into a clamping position, a second spring element via which the clamping element can be loaded with a first force, a third spring element, via which the clamping element can be loaded with a second force, and an axially movable actuating device by which the centering clamping member and the clamping element can be moved into a release position against the force of the first and third spring elements.
The device according to the invention allows automatic clamping and centering of a workpiece with defined forces and reliably prevents the occurrence of clamping errors. The device further has the advantage that it is kept play-free by means of the spring elements and therefore no oscillations occur which impair the accuracy of the unbalance measurement.